Demolition hammer and/or hammer-drill with a percussion device suitable for freely striking clamped objects

ABSTRACT

The invention relates to a demolition hammer and/or hammer drill comprising a percussion generating device having a percussion piston moving axially back and forth. In a main percussion mode, the percussion piston impinges upon a tool which can move along a limited axial path straight into a main percussion direction. In a free percussion mode, the percussion piston indirectly impinges upon a percussion changing device, and by means thereof upon the tool, in a free percussion direction opposite to the main percussion direction. The impact surface of a tool shaft and the impact surface of a free percussion ram are placed opposite an impact surface of the percussion piston in such a way that the free percussion ram is removed from the effective area of the percussion piston by a return spring in the main percussion position. Under the effect of the return spring, the percussion ram is simultaneously supported on a gear member of the percussion changing device, which projects with an extension into the motion path of a stop surface provided in the tool. The stop surface reaches the extension when it has moved away from the effective area of the percussion piston in an idle running state of the tool shaft. If the movement of the tool is continued, the stop surface impinges upon the gear member against the effect of the return spring and relocates the free percussion ram in the effective area of the percussion piston by overcoming the spring force.

[0001] The invention relates to a demolition hammer and/or hammer drillin accordance with the preamble of claim 1.

[0002] When working with demolition hammers and hammer drills it isfrequently the case that the tool, e.g. a chisel or a drilling tool,becomes fixedly wedged or jammed in the rock. Whilst, after gaining somepractice at drilling using the hammer drill, it is still possible inmost cases to prevent the tool from becoming jammed, during demolitionwork using a large demolition hammer it is not always possible even forthe person skilled in the art to prevent the chisel from becoming wedgedor jammed.

[0003] This has been remedied by means of a percussion device which isdisclosed in DE 197 31 732 A1. If the tool becomes jammed, the personoperating the demolition hammer or hammer drill is able to switch on afree percussion device which serves to deflect the drive force, so thatit influences the tool in the opposite direction to the main percussiondirection, whereby—as tests have shown—the jammed tool can be releasedwith a small number of impacts.

[0004] In the case of the known percussion device, a push lever is to beactuated on each occasion by the operator when the tool becomes jammed,whereby a percussion changing device can be activated. Furthermore,difficulties are to be expected in the sealing arrangement.

[0005] Therefore, it is the object of the invention to design ademolition hammer and/or hammer drill of the generic type such that itis possible—without any operating effort on the part of the operator—toswitch in a simplified manner between the normal mode and the freepercussion mode, wherein consideration is to be afforded to a principle,which is favourable in terms of strength, and to effective sealingcapability.

[0006] In accordance with the invention the object is achieved by meansof a demolition hammer and/or hammer drill in accordance with claim 1.Advantageous further developments of the invention are provided in thedependent claims.

[0007] In the case of the demolition hammer and/or hammer drill inaccordance with the invention (also referred to hereinunder as a“hammer”, a switch is made from the main percussion state to the freepercussion state, if the demolition hammer and/or the hammer drill ispulled away from the material being worked when the tool becomes jammed.The hammer is pulled away intuitively by the operator, if he establishesthat the tool has become jammed. As it is then the case that a switch ismade to the free percussion state, the operator does not have to actuateany further devices, such as e.g. push-levers or the like whichsignificantly reduces operational effort.

[0008] By pulling the hammer away from the rock being worked, a pullingforce is generated in or on the tool which is detected by a forcedetection device and is changed respectively into a different physicalvariable such as e.g. a path. The force detection device cooperates witha switching device in such a manner that—if the pulling force exceeds apredetermined threshold value—the switching device performs a switch tothe free percussion state. This means that the operator must pull on thehammer at least with a specific predetermined force, so that the switchis made to the free percussion state.

[0009] In an advantageous manner, the force detection device and theswitching device form part of the percussion changing device. If thecomponents of the percussion changing device can be used in this mannerfor several functions simultaneously, further components which aresensitive or susceptible to maintenance under certain circumstancesbecome superfluous.

[0010] In the case of a particularly advantageous embodiment of theinvention, the pulling force effective on the tool is proportional to anaxial displacement of the tool relative to the rest of the hammer. Thismeans that the force detection device is able to measure the pullingforce by means of the axial displacement of the tool, e.g. against theeffect of a spring. If the axial displacement of the tool exceeds apredetermined path, it is possible to conclude from this that thepredetermined threshold value of the pulling force has also beenexceeded. By detecting the axial displacement against the effect of aspring of the tool, it is not necessary to determine the pulling forcewhich is actually effective on the tool. On the contrary, the axialdisplacement can be relayed via corresponding mechanisms directly to theswitching device and can be used for the purpose of effecting a switchover to the free percussion state.

[0011] However, in the case of other embodiments of the invention thepulling force which is effective on the tool can also be determined bymeans of suitable force sensors, if there is no axial displacement ofthe tool relative to the rest of the hammer. The signals of the forcesensors are then to be relayed in a suitable manner to the switchingdevice.

[0012] In the case of a preferred embodiment of the invention, thepercussion changing device comprises a first part, which in the freepercussion state can be displaced into the effective region of thepercussion piston, and comprises a second part which can be moved intopositive-locking contact with a stop provided on the tool, wherein amovement of the tool can be transmitted via the second part to the firstpart of the percussion changing device, in order to move it into theeffective region of the tool. As a consequence, the percussion changingdevice establishes an operative connection between the percussion pistonand the stop on the tool, so that the percussion piston can influencethe tool indirectly in the free percussion direction.

[0013] In the case of an advantageous embodiment, the percussionchanging device comprises a free percussion ram, which can be moved intothe effective region of the percussion piston of the percussive tool,and an extension which engages behind a collar of the tool. Thepercussion changing device is pretensioned by means of a spring in sucha manner that in the main percussion state the free percussion ram isnot located in the effective region of the percussion piston.

[0014] If in the free percussion state, i.e. when the tool is jammed,the demolition hammer and/or hammer drill is lifted from the materialbeing worked, the collar of the tool presses against the extension, sothat the percussion changing device and thus the free percussion ram aredisplaced against the effect of the return spring. As a consequence, thefree percussion ram moves into the effective region of the percussionpiston and can be influenced during a subsequent impact. The effect ofthe impact of the percussion piston is transmitted via the freepercussion ram and the percussion changing device to the extension andthus ultimately to the collar of the tool in the free percussiondirection, opposite the main percussion direction.

[0015] In so doing, it is particularly advantageous if the percussionpiston influences the free percussion ram in the free percussion statein the main percussion direction as this serves to maintain thepercussion direction of the percussion piston.

[0016] In the case of a preferred embodiment, the demolition hammerand/or hammer drill is subdivided into a drive unit and a tool unitwhich each comprise a housing and can be mutually coupled bymechanically connecting the housings, wherein the coupling region of thedrive unit which is open with respect to the tool unit is provided withthe impact surface of the percussion piston, opposite to which in thecoupling region of the tool unit lie the impact surface of the toolshaft and the impact surface of the free percussion ram such that in themain percussion state the free percussion ram is removed by the returnspring from the effective region of the percussion piston and at thesame time is supported under the effect of this return spring on a gearmember which is associated with the percussion changing device and whichprotrudes with an extension into the movement path of the stop surfaceprovided on the tool, wherein the stop surface reaches the extension, ifduring an idle running state the tool shaft has been removed from theeffective region of the percussion piston, so that as this movement ofthe tool continues the stop surface influences the gear member againstthe effect of the return spring and displaces the free percussion raminto the effective region of the percussion piston when this springforce is overcome.

[0017] As long as the hammer is pressed against the material beingworked, the tool is consequently urged at the same time into theeffective region of the percussion piston, whereas the return springkeeps the free percussion ram out of this effective region. If thehammer is pulled back, the tool slides out of the effective region ofthe percussion piston until it lies against the extension. The device isthen located in the idle running state. If the tool becomes jammed andthe device is pulled further back, the pulling force increases and movesthe extension against the effect of the return spring and thereby urgesthe free percussion ram into the effective region of the percussionpiston, so that the percussion piston then performs impacts via theextension on to the tool in the opposite direction to the mainpercussion direction. This state is called the free percussion state. Assoon as the tool is released, the return spring re-establishes theoriginal state and the device is then located in the idle running state,until it is pressed against the material once again.

[0018] The change between the operating states is performedautomatically. A mechanical connection is provided between the driveunit and the tool unit merely by virtue of the connection of theirhousings on both sides. The impacts of the percussion piston on to thetool and, where appropriate, the necessary release impacts on to thefree percussion ram and the switch between the various operating statesdo not require any additional connection between the two units. Uponrelease of the coupling connection, they can be separated quickly andconveniently from each other, maintained separately and put backtogether again as quickly and conveniently. It is possible to connecttool units, which are adapted to suit different tools, to the drive unitif the connection dimensions are respected only in the connection regionand the impact surfaces of the tool and of the free percussion ram lieopposite the percussion piston. This means that there is extensive scopewith regard to the respectively expedient design of the percussionchanging device.

[0019] In a manner which is known per se, the stop is preferably acollar which widens the cross-section of the tool.

[0020] According to an advantageous embodiment, the gear member is adeflecting lever which is mounted on the housing in such a manner as tobe able rotate about a first axle, wherein according to a furtherembodiment the gear member is allocated a blocking device which issuitable for limiting its angle of rotation between two limit positionssuch that the extension always protrudes into the path of the stop andthe free percussion ram is located in the one limit position in theeffective region of the percussion piston and is located in the otherlimit position just outside this effective region.

[0021] Preferably, the blocking device is a latching and unlatchinglever which is mounted on the housing in such a manner as to be able torotate about an axle, which is in parallel with the axle of rotation ofthe gear member and has a spaced interval therefrom, and can be fixed bymeans of a latch connection in a blocking and locking position in itsangular position relative to the housing, that a spigot which is formedon the gear member engages into a connecting link which is provided onthe latching and unlatching lever and which in the blocking and latchingposition of said lever limits the angle of rotation of the gear memberbetween the two limit positions, whereas upon overcoming the latchconnection the latching and unlatching lever can be pivoted into anunlatching position, in which the connecting link has pivoted the gearmember to a position, in which the extension is located outside themovement path of the stop on the tool.

[0022] Another very expedient embodiment is one in which the unlatchinglever consists of two limbs which are axially spaced apart from eachother and are connected at their ends remote from the axle by means of across-piece and which, for mounting in the housing, are provided in eachcase with an axle stub on their mutually remote outer sides, and thatthe gear member engages into the intermediate space between the twolimbs, whereas its axle is located outside the pivot region of thelatching and unlatching lever.

[0023] The invention will now be explained in detail with reference tothe description hereinunder of a preferred exemplified embodiment of theinvention as illustrated in the Figures, in which

[0024]FIG. 1a shows an axial sectional view of the tool unit with thetool inserted in the percussion position (main percussion state),

[0025]FIG. 1b shows a sectional view in parallel with the axialsectional view as shown in FIG. 1a and slightly offset with respect tothe sectional view of FIG. 1a in the direction of the viewer,

[0026]FIG. 1c shows a perspective view of the tool unit in the mainpercussion state,

[0027]FIG. 2a shows an illustration, corresponding to FIG. 1a, of theidle running state,

[0028]FIG. 2b shows an illustration, corresponding to FIG. 1b, of theidle running state,

[0029]FIG. 2c shows a perspective view of the tool unit in the idlerunning state,

[0030]FIG. 3a shows an illustration, corresponding to FIG. 1a, in thefree percussion position (free percussion state),

[0031]FIG. 3b shows an illustration, corresponding to FIG. 1b, in thefree percussion position,

[0032]FIG. 3c shows a perspective view of the tool unit in the freepercussion position,

[0033]FIG. 4a shows an illustration, corresponding to FIG. 1a, in theopen position with the tool removed,

[0034]FIG. 4b shows an illustration, corresponding to FIG. 1b, in theopen position with the tool removed, and

[0035]FIG. 4c shows a perspective view of the tool unit in an openposition with the tool removed.

[0036] The Figures all illustrate a section of an inventive demolitionhammer and/or hammer drill in various states or views. Therefore, inorder to explain the structure reference is made hereinunder to allFigures simultaneously.

[0037] The drawing illustrates only the tool unit, which is designatedoverall by the reference numeral 10, of a demolition hammer and/orhammer drill. The drive unit which contains a percussion generatingdevice in a design which is known per se, e.g. a pneumatic springpercussive tool, has been omitted for the purpose of simplification. Itcontains e.g. a crank drive, which is driven by an electromotor, and adrive piston which can be moved in an axial direction by means of thiscrank drive and which can be moved axially in a hollow-cylindricalrecess of a percussion piston. The percussion piston is disposed in anaxially movable manner in a housing of the drive unit such that its endsurface which serves to transmit force to the tool in a main percussiondirection and is defined as the impact surface is located in the regionof a housing orifice which faces the tool unit 10 which is to beconnected to the drive unit. Where appropriate, it is also possible todispose a riveting die, which serves as an intermediate element, betweenthe percussion piston and the tool. Essentially all types of percussivetools are suitable for the application of the invention.

[0038] The operating states are distinguished as follows: A mainpercussion state, in which the hammer operates in a conventional manner,i.e. impacts are exerted upon a tool; an idle running state, in whichthe drive of the does continue to operate but no impacts are exerted onto the chisel by the percussion generating device; and a free percussionstate, in which impacts are exerted on to the tool in a free percussiondirection which is opposite to the main percussion direction, i.e. awayfrom the material being worked.

[0039] The tool unit comprises a housing 12 which is penetrated by acentral guide channel 14 for the shaft 16 of a tool 18, e.g. a chisel,which comprises a hexagonal cross-section. On its coupling end 20 whichis allocated to the drive unit, the housing 12 is provided with acylindrical projection 22 which contains on its outer side an annulargroove 24 for the purpose of receiving a seal. This projection 22 can beinserted into an allocated receiving device on the housing of the driveunit [not illustrated]. A flange 28, which is provided with bores 26, onthe housing 12 of the tool unit serves to connect the tool unit 10 andthe drive unit in a mechanical manner. The connection can be establishedquickly and conveniently by means of two screws. Further measures arenot required for the purpose of coupling the two units.

[0040] The tool 18 is provided with a collar 30, for the reception ofwhich the guide channel 14 in the end portion of the housing 12 remotefrom the projection 22 is provided with a cross-sectional widening 32.The length of this is dimensioned in such a manner that the tool 18 isprevented from moving further in the direction of the projection 22 byvirtue of the fact that the collar 30 lies against the end of thecross-sectional widening 32, if the shaft 16 in the region of theprojection 22 protrudes out of the housing 12 to such an extent into themovement region of the percussion piston [not illustrated] that thepercussion piston is able to transmit its percussion movement to thetool 18. This situation which is assumed in the main percussion stateand is defined as the percussion position is illustrated in FIGS. 1a to1 c.

[0041] The end of the housing 12 facing the drive unit is provided witha trough-like recess 34, in the centre of which the guide channel 14opens out. Adjacent to the guide channel 14, a guide channel 36 for afree percussion ram 38 opens out. This guide channel 36 approaches theguide channel 14 at an acute angle in the direction of the recess 34. Inits guide portion 40 which opens out in the recess 34, the guide channel36 surrounds the cylindrical free percussion ram 38 which in its endportion 42 remote from the recess 34 comprises an enlarged cross-sectionwhich serves to form a shoulder 44. Formed in the end portion 42 is alongitudinal slot 46 through which passes a pin 48 which is attached inthe housing 12, whereby the free percussion ram 38 is prevented fromrotating. The end of the free percussion ram 38 which is remote from therecess 34 comprises a contact surface 50 which extends in an inclinedmanner with respect to the axis of the ram and which is alignedapproximately at a right angle with respect to the axis of a piston 52which is guided in a movable manner in an axial direction in the housing12 and which is part of a percussion changing device 54 which isdisposed substantially between two mutually parallel flanges 56 a and 56b which are disposed on the housing 12.

[0042] The percussion changing device 54 further comprises two congruentlimbs 58 a and 58 b of a latching and unlatching lever 58 which aremutually connected by means of a cross-piece 60 at the free end of thelever. The limbs 58 a and 58 b are mounted in each case in a rotatablemanner on one of the flanges 56 a and 56 b respectively by means of axlestubs 62 which are disposed in a coaxial manner with respect to eachother. Disposed between the two limbs 58 a and 58 b is a gear member orcurve piece 64 which serves as a deflecting lever and locking bar andwhich is mounted outside the pivot region, which is utilised by thelatching and unlatching lever 58, by virtue of an axle 66 on the flanges56 a and 56 b which is in parallel with the axle stubs 62. A spigot 68which is connected to the curve piece 64 engages in connecting links 70which are formed congruently on the limbs 58 a and 58 b. The curve piece64 is also provided with an extension 72 which protrudes into themovement path of the collar 30 on the tool 18 in the percussion positionillustrated in FIGS. 1a to 1 c, wherein the collar 30 does not, however,reach as far as the extension 72 during the percussion mode of the tool18.

[0043] Between the end of the guide portion 40, which is remote from therecess 34, and the shoulder 44, a helical compression spring 74 whichserves as a return spring surrounds the free percussion ram 38 and drawsthe free percussion ram 38 out of the recess 34 and presses with itscontact surface 50 against the piston 52 which for its part is supportedon the curve piece 64 and transmits thereto a torque which in FIGS. 1aand 1 b acts in an anti-clockwise direction and which serves to supportthe spigot 68 in the connecting links 70 and thus on the latching andunlatching lever 58. The latching and unlatching lever 58 is provided onits periphery with a cam-like projection 76 which lies against anelastic sleeve 78 which surrounds the axle 66. The resistance of thissleeve 78 is sufficient to fixedly hold the latching and unlatchinglever 58 in the position shown in FIGS. 1a to 1 c, whereby the curvepiece 64 also retains its position and the extension 72 is held in thepath of the collar 30.

[0044] The latching and unlatching lever 58 is additionally fixed byvirtue of the fact that the effect of the helical compression spring 74causes the piston 52 to be pressed against the curve piece 64. The curvepiece 64 supports the spigot 68 and urges it to the position shown inFIG. 1b in the connecting link 70 against the latching and unlatchinglever 58. By reason of the force which is introduced by the spigot 68and is effective about the axle 66, a torque is generated about the axlestub 62 which presses the latching and unlatching lever 58 about theaxle stub 62 to the position shown in FIG. 1a against a stop 82 to beexplained hereinunder. Therefore, the helical compression spring 74 alsoindirectly causes the latching and unlatching lever 58 to be fixedlyheld in the position shown in FIGS. 1a to 1 c.

[0045] In the percussion position as shown in FIGS. 1a to 1 c, asachieved in the main percussion state, it is only the tool 18 which isinfluenced by the percussion piston.

[0046] If the pressure on the hammer is removed and the hammer is pulledback from the material being worked, when the device is held in theconventional manner with the tip of the tool pointing downwards the tool18 will slide a certain length out of the housing 12 until the collar 30is stopped by the extension 72. This is the idle running position asshown in FIGS. 2a to 2 c, in which the tool 18 is not influenced by thepercussion piston and the free percussion device also does not becomeeffective.

[0047] Furthermore, in the idle running position it is possible for thepercussive tool [not illustrated] to change over in a known manner to anidle running state, in which the percussion piston does not perform anyimpacts.

[0048] If, after achieving the idle running state, the operator pullsharder on the hammer and the tool has become jammed in the materialbeing worked, the tool cannot take part in the withdrawal movement ofthe hammer. The pulling force exerted by the operator upon the devicebecomes effective at the extension 72 and seeks to pull the tool 18 atits collar 30 out of its jammed position. By reason of the resistance ofthe jammed tool 18, the force of the helical compression spring 74 isovercome. In the drawing, the curve piece 64 rotates in a clockwisedirection, lifts the piston 52 and presses the free percussion ram 38against the effect of the helical compression spring 74 into themovement region of the percussion piston, whereby the impacts thereofare deflected by the curve piece 64, which represents a two-armed lever,on the extension 72 in the opposite direction and, by way of the collar30, the impacts influence the tool 18 in the opposite direction to themain percussion direction in a free percussion direction.

[0049] Depending upon the behaviour of the operator, it is possible tochange directly from the main percussion state to the free percussionstate or to change indirectly from the main percussion state via theidle running state to the free percussion state. If the operator quicklylifts the hammer from the rock being worked when the tool is jammed,there is no time for the percussive tool to change to the idle runningstate by the displacement of the percussion piston. On the contrary,from one stroke to another the percussion piston is not presented withthe tool shaft 16 but rather with the free percussion ram 38, so thatthe percussion mode is continued (however at this time as the freepercussion mode).

[0050] In contrast, if the operator slowly lifts the hammer from therock being worked when the tool is jammed, the idle running stateillustrated in FIGS. 2a to 2 c is set, in which neither the tool shaft16 nor the free percussion ram penetrates into the effective region ofthe percussion piston. Subsequently, the percussion piston slides so farforwards into the region of the recess 34 that the percussive tool [notillustrated] changes to the idle running state. As the hammer is pulledfurther with respect to the tool 18 which is jammed, the free percussionram 38 pushes the percussion piston back into the percussive tool whichserves then to start up the percussion mode but this time in the freepercussion mode.

[0051] This situation which is achieved in the free percussion state andis also defined as the free percussion position is illustrated in FIGS.3a to 3 c.

[0052] As soon as the tool 18 is released, the pulling force at theextension 72 diminishes and the helical compression spring 74 urges thefree percussion ram 38 back out of the effective region of thepercussion piston, so as to restore the idle running position as shownin FIGS. 2a to 2 c.

[0053] If the tool 18 is to be removed from the tool unit 12, a force isexerted in an anti-clockwise direction upon the latching and unlatchinglever 58, whereby the cam-like projection 76 pushes past the elasticsleeve 78 and the latching and unlatching lever 58 can be moved to theposition shown in FIGS. 4a to 4 c. The connecting links 70 serve toentrain the spigot 68 and pivot the curve piece 64 in such a manner inan anti-clockwise direction that the extension 72 is pivoted upwards outof the path of the collar 30 and the tool 18 can be removed. Thissituation as shown in FIGS. 4a to 4 c is defined as the openingposition.

[0054] The percussion changing device 54 serves to deflect the impacts,which are exerted upon it in the main percussion direction by thepercussion piston, in the opposite direction which is also defined asthe free percussion direction, so that the impacts can be transmitted bymeans of the extension 72 to the collar 30 of the tool 18 in the freepercussion direction. For this purpose, the percussion changing device54 comprises inter alia the free percussion ram 38 with the helicalcompression spring 74, the piston 52, the curve piece 64 and theextension 72.

[0055] The percussion changing device 54 also serves to detect the forceacting upon the tool which is jammed, if the operator attempts to pullthe hammer away. Therefore, the percussion changing device comprises atype of force detection device, with the aid of which it is possible tointroduce a pulling force via the extension 72 into the tool 18 whichforce simultaneously effects a displacement of the free percussion ram38 against the helical compression spring 74. The displacement of thefree percussion ram 38 is thus proportional to the pulling forcegenerated by the operator.

[0056] Furthermore, the percussion changing device 54 comprises aswitching device, as it enables a switch over to the free percussionstate by virtue of the corresponding displacement of the free percussionram 38 to the percussion region of the percussion piston.

[0057] Skilled dimensioning of the helical compression spring 74 allowsthe force detection device to be configured in such a manner that e.g.it is as yet insufficient simply to lift the hammer with the tool fromthe rock being worked and therefore to exert the weight force of thetool 18 upon the force detection device, in order to effect a switchfrom the main percussion state to the free percussion state by means ofthe switching device. In particular, this can be achieved by virtue ofthe fact that the helical compression spring 74 is prestressed, so thatthe helical compression spring 74 is not deformed until thepretensioning force representing a threshold value is exceeded. Thethreshold value should be such that even in the case of heavy tools itis not possible to effect a switch over to the free percussion state byreason of the weight of the tool alone. Only if it is established thatthe operator has pulled harder on the hammer can a switch be performed.

[0058] Essentially, it is also possible in the case of other embodimentsof the invention to provide the force detection device, which is to beused to detect the pulling force applied by the operator on the hammer,and the switching device for the purpose of switching between the mainpercussion state and the free percussion state as separate devices, i.e.in addition to the percussion changing device 54.

[0059] The curve piece 64 comprises a specially formed lateral contour80 which adjoins the extension 72. The contour 80 serves as a slidingsurface for the purpose of supporting the end face of the piston 52which is directed towards the curve piece 64 and the different radialdistance of the end face with respect to the axle 66 ensures that thepiston 52 and thus also the free percussion ram 58 are axially displacedaccordingly. The different positions of the curve piece 64 having thecontour 80 and thus of the piston 52 are evident in particular in FIGS.1a, 3 a and 4 a.

[0060] As shown in particular in FIG. 3a, the curve piece 64 is pivotedby the collar 30 about the axle 66 to the extent until parts of thecontour 80 lie against the stop 82 which is formed on the housing 12 ofthe tool unit and represents an inclined surface. This serves toestablish a secure end position for the curve piece 64 which alsoensures that the extension 72 remains in contact with the collar 30 ofthe tool 18. As already explained above, the stop 82 also serves to fixthe latching and unlatching lever 58 in the position illustrated in FIG.1b.

1. Demolition hammer and/or hammer drill having a percussion generatingdevice which comprises a percussion piston which can be moved axially ina reciprocating manner, wherein in a main percussion state thepercussion piston influences a tool shaft (16) of a tool (18), which ismovable over a limited axial path, directly or via an intermediateelement in a main percussion direction, whereas, in a free percussionstate, the percussion piston does not influence the tool shaft (16)directly, but rather influences a percussion changing device (54) and byway of this influences the tool (18) indirectly in a free percussiondirection which is opposite to the main percussion direction, andwherein it is possible to switch over from the main percussion state tothe free percussion state, if the tool (18) is jammed in a materialbeing worked, characterised in that a force detection device is providedfor the purpose of detecting a pulling force which acts upon the tool(18) jammed in the material being worked and which is generated bypulling the demolition hammer and/or hammer drill away from the tool(18), and that the force detection device cooperates with a switchingdevice for the purpose of switching over from the main percussion stateto the free percussion state, wherein the switch over to the freepercussion state is performed if the pulling force exceeds apredetermined threshold value.
 2. Demolition hammer and/or hammer drillas claimed in claim 1, characterised in that the force detection deviceand the switching device form part of the percussion changing device(54).
 3. Demolition hammer and/or hammer drill as claimed in claim 1 or2, characterised in that the pulling force effective on the tool (18) isin specific proportion to an axial displacement of the tool (18)relative to the rest of the demolition hammer and/or hammer drill, andthat the tool (18) is displaced in the free percussion state withrespect to the main percussion state over the distance of an axial path.4. Demolition hammer and/or hammer drill as claimed in claim 2 and 3,characterised in that a first part (38) of the percussion changingdevice (54) can be displaced in the free percussion state into theeffective region of the percussion piston by means of tool (18) whichcan be pulled along the axial path out of the demolition hammer and/orhammer drill.
 5. Demolition hammer and/or hammer drill as claimed inclaim 3 or 4, characterised in that a second part (72) of the percussionchanging device (54) is formed in such a manner that it can be movedinto positive-locking contact with a stop (30) provided on the tool(18), such that a movement of the tool (18) can be transmitted via thesecond part to the first part of the percussion changing device (54). 6.Demolition hammer and/or hammer drill as claimed in claim 4 and 5,characterised in that in the main percussion state a free percussion ram(38) which forms the first part of the percussion changing device (54)is removed from the effective region of the percussion piston by theeffect of a return spring (74), and at the same time the percussionchanging device (54) is pretensioned under the effect of the returnspring (74) in such a manner that an extension (72) which is associatedwith the percussion changing device and forms the second part protrudesinto the movement path of the stop (30) which is provided on the tool(18), and that in the free percussion state by reason of its changedaxial position the stop (30) of the tool (18) influences the percussionchanging device (54) against the effect of the return spring (74) anddisplaces an impact surface of the free percussion ram (38) into theeffective region of the percussion piston when the spring force isovercome.
 7. Demolition hammer and/or hammer drill as claimed in claim6, characterised in that the percussion piston influences the freepercussion ram (38) in the free percussion state in the main percussiondirection.
 8. Demolition hammer and/or hammer drill as claimed in claim6 or 7, characterised in that the percussion hammer and/or hammer drillis subdivided into a drive unit and a tool unit (10) which each comprisea housing (12) and can be mutually coupled by mechanically connectingthe housings; the coupling region, which is directed towards the toolunit, of the drive unit is provided with the impact surface of thepercussion piston, opposite to which in the coupling region of the toolunit lie the impact surface of the tool shaft (16) and the impactsurface of the free percussion ram (38) such that in the main percussionstate the free percussion ram (38) is removed from the effective regionof the percussion piston by means of the return spring (74) and at thesame time is supported under the effect of the return spring (74) on agear member (64) of the percussion changing device (54), which gearmember protrudes with the extension (72) into the movement path of thestop (30) which is provided on the tool (18); and that the stop (30)reaches the extension (72) if, in an idle running state, the tool shaft(16) has been removed from the effective region of the percussionpiston, so that as this movement of the tool (18) is continued, the stop(30) influences the gear member (64) against the effect of the returnspring (74) and displaces the free percussion ram (38) into theeffective region of the percussion piston when the spring force isovercome.
 9. Demolition hammer and/or hammer drill as claimed in any ofclaims 1 to 8, characterised in that the stop is a collar (30) whichwidens the cross-section of the tool (18).
 10. Demolition hammer and/orhammer drill as claimed in any of claims 6 to 9, characterised in thatthe gear member (64) is a deflecting lever which is mounted on thehousing 12 so as to be able to rotate about a first axle (66). 11.Demolition hammer and/or hammer drill as claimed in any of claims 6 to10, characterised in that the gear member (64) is allocated a blockingdevice (58) which is suitable for limiting the angle of rotation of thegear member (64) between two limit positions such that the extension(72) always protrudes into the path of the stop (30) and the freepercussion ram (38) is located in one limit position in the effectiveregion of the percussion piston and is located in the other limitposition just outside this effective region.
 12. Demolition hammerand/or hammer drill as claimed in claim 11, characterised in that theblocking device is a latching and unlatching lever (58) which is mountedon the housing (12) in such a manner as to be able to rotate about anaxle (62), which is in parallel with the axle of rotation (66) of thegear member (64) and has a spaced interval therefrom, and said latchingand unlatching lever can be fixed by means of a latch connection (76,78) in a blocking and latching position in its angular position relativeto the housing (12), and that a spigot (68) which is formed on the gearmember (64) engages into a connecting link (70) which is provided on thelatching and unlatching lever (58) and which in the blocking andlatching position of said lever limits the angle of rotation of the gearmember (64) between the two limit positions, whereas after the latchconnection (76, 78) has been overcome the latching and unlatching lever(58) can be pivoted to an unlatching position, in which the connectinglink (70) has pivoted the gear member (64) to a position, in which theextension (72) is located outside the movement path of the stop (30)provided on the tool (18).
 13. Demolition hammer and/or hammer drill asclaimed in claim 12, characterised in that the latching and unlatchinglever (58) consists of two limbs (58 a, 58 b) which are axially spacedapart from each other and are connected at their ends remote from theaxle (62) by means of a cross-piece (60) and which, for mounting in thehousing (12), are provided in each case with an axle stub (62) on theirmutually remote outer sides, and that the gear member (64) engages intothe intermediate space between the two limbs (58 a, 58 b), whereas itsaxle (66) is located outside the pivot region of the latching andunlatching lever (58).
 14. Demolition hammer and/or hammer drill asclaimed in any of claims 8 to 13, characterised in that the drive unitand the tool unit are mutually connected in one piece.
 15. Demolitionhammer and/or hammer drill as claimed in any of claims 1 to 14,characterised in that between the main percussion state and the freepercussion state, an idle running state is set for at least a shortperiod of time with the tool (18) in the idle running position. 16.Demolition hammer and/or hammer drill as claimed in any of claims 1 to15, characterised in that the percussion piston influences thepercussion changing device (54) in the free percussion state in the mainpercussion direction.